1. Field of the Invention
This invention relates to a light-quantity adjusting device for use in a still camera, a video camera, etc.
2. Description of the Related Art
The non-interchangeable lens type camera, which is called the compact camera, generally has a light-quantity adjusting device serving both as the shutter and the diaphragm. In recent years, the use of the stepping motor as a drive source for the light-quantity adjusting device has become the mainstream. Meanwhile, the diaphragm for the interchangeable lens of the single-lens reflex camera, too, employs, in some cases, the stepping motor as a drive source.
To adapt such a stepping motor to the drive source of an electromagnetically-driven light-quantity adjusting device, as disclosed in Japanese Laid-Open Utility Model Applications No. Sho 60-141682, No. Sho 60-141683, No. Sho 60-140934, etc., the stepping motor is composed of an arcuate base plate, a rotor having its outer periphery magnetized to quadripole, a pair of stators of almost letter "U" shape and a pair of coils for exciting the respective stators.
Another form of the light-quantity adjusting device is also known in Japanese Laid-Open Patent Applications No. Sho 62-240942, No. Hei 1-164258, etc., where coils each have one piece of iron core in parallel with the shaft of the rotor and are wound around the iron cores, and a pair of stators each have two magnetic pole members positioned in a plane perpendicular to the rotor shaft and each having a plurality of magnetic pole elements confronting the outer periphery of the rotor.
By the way, in the light-quantity adjusting device with the stepping motor used as a drive source, in the case of, for example, the single-lens reflex camera, the actual size of opening of the diaphragm aperture is determined by setting a certain phase at which electric power starts being supplied to the stepping motor and counting the number of steps by which the rotor is rotated. For this reason, to attain a high accuracy of aperture control, a smaller angle of rotation of the rotor per step is desired.
Meanwhile, to avoid premature consumption of the battery serving as the electric power source, on account of a higher probability of making relatively long exposures, a high percentage of the period in which the current supply to the motor is cut off is more advantageous. On this account, it is desired to increase the number of stop positions, at which the rotor comes to stably stop due to the cogging torque, per revolution of the rotor.
In the conventional examples such as those disclosed in the aforesaid Japanese Laid-Open Utility Mode Applications No. Sho 60-141682, 60-141683 and 60-140934, however, although the number of pulses required for rotating the rotor by one revolution by using the known 1,2-phase current supply is 16, the number of the stable stop positions caused by the cogging torque is only four as it is equal to the number of magnetized poles. For this reason, even when making a relatively long exposure, the current supply to the coil must continue till the end of the exposure. Otherwise, one could not get the high accuracy of diaphragm aperture control. Hence, there is a problem of a very fast exhaustion of the battery serving as the electric power source.
Also, in the electromagnetically-driven light-quantity adjusting device such as those disclosed in the Japanese Laid-Open Patent Applications No. Sho 62-240942 and No. Hei 1-164258, the stators are disposed in a three-dimensional relation. Therefore, accurate arrangement of the phase of each magnetic pole is difficult to perform. Hence, there is a drawback that the accuracy of the size of opening of the diaphragm aperture is difficult to stably obtain at a high level.
Further, in the electromagnetically-driven light-quantity adjusting device disclosed in the Japanese Laid-Open Patent Application No. Hei 1-164258, the number of positions at which the rotor stops stably due to the cogging torque in response to the termination of current supply is equal to 1/2 of the number of positions at which the rotor can stop under the condition that the current supply continues. This is acceptable as the above number is relatively great. However, when the termination of current supply coincides with occurrence of an unstable stop position, it is impossible to specify the direction in which the cogging torque works, or determine whether the diaphragm is to move toward the minimum size of aperture opening by an amount equal to one step, or toward the full open aperture by that amount. As a result, there is a high possibility of occurrence of an error of .+-.1 step. Hence, a problem arises that the accuracy with which to control the size of aperture opening cannot be assured with high reliability under the condition that the current supply is cut off.